Photocathode manufacture

ABSTRACT

A photocathode structure containing a photocathode material, comprising a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material, and, an epitaxial layer of photocathode material located on a first said major surface of said crystal, the thickness of said layer of photocathode material being of the order of the diffusion length of electrons therein and at least part of a second said major surface of the gallium indium phosphide plate being substantially free from contact by solid material.

United States Patent 1 Gowers June 10, 1975 PHOTOCATHODE MANUFACTURE [73] Assignee: U.S. Philips Corporation, New

York, NY.

22 Filed: Nov. 23, 1973 211 App]. No.: 418,295

[30] Foreign Application Priority Data 3,696,262 10/l972 Antypas 313/94 3,699,401 10/1972 Tietjfn et al. 317/235 NX Primary Examiner.1ames W. Lawrence Assistant Examiner-E. R. La Roche Attorney, Agent, or FirmFrank R. Trifari; Leon Nigohosian [5 7 ABSTRACT A photocathode structure containing a photocathode material, comprising a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material, and, an epitaxial layer of photocathode material located on a first said major surface of said crystal, the thickness of said layer of photocathode material being of the order of the diffusion length of electrons therein and at least part of a second said major surface of the gallium indium phosphide plate being substantially free from contact by solid material.

6 Claims, 1 Drawing Figure Aug. 21, 1973 United Kingdom 54387/73 [52] U.S. Cl. 313/94; 313/373; 357/30; 357/31 [51] Int. Cl H01j 39/16; HOlj 39/06 [58] Field of Search 313/65 R, 65 AB, 68 R, 313/94, 95, 102, 103, 108 D; 317/235 N [56] References Cited UNITED STATES PATENTS 3,478,213 11/1969 Simon et a] 313/103 X 3,575,628 4/1971 Word 313/95 a k 6 r o PATENTEDJUH 10 1975 PHOTOCATHODE MANUFACTURE This invention relates to=a method'of manufacturing a photocathode which includes a single crystal layer of photocathode material such as p-type gallium arsenide the thickness of which is of the order of the diffusion length of electrons therein.

It is necessary that the thickness of gallium arsenide photocathode of the transmission type-beef theorder of the diffusion length of electrons therein because, if their thickness were appreciably greater, electrons excited by the absorption of input radiation would not be able to diffuse to the emissive surface of the photocathode without substantial recombination occurring. In practice this means that the thickness of the layer must be only a few microns. Because a free-standing layer of gallium arsenide with this order of thickness would be extremely fragile, it is desirable to provide the layer on a substrate to give it mechanical strength. This subof gallium aluminium arsenide for this purpose. A layer,

of the gallium aluminium arsenide is first grown on a gallium arsenide crystal, after which a thin p-type layer of gallium arsenide is grown on the free surface of the gallium aluminium arsenide. The original gallium arsenide crystal is finally completely removed to leave a thin layer of p-type gallium arsenide on gallium aluminium arsenide.

It is an object of the invention to provide an alternative material on which the thin layer of p-type gallium arsenide is provided.

According to one aspect the invention provides a method of manufacturing a photocathode which includes a single crystal layer of photocathode material (which may be p-type gallium arsenide) the thickness of which is of the order of the diffusion length of elec-.

trons therein, said method comprising the steps of:

a. growing a single crystal layer of gallium indium phosphide by an epitaxial technique on a single crystal substrate of said photocathode material, the phosphide having relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material,

b. subsequently growing said layer of photocathode material by an epitaxial technique on the free surface of the gallium indium phosphide layer, and

c. subsequently removing at least part of said substrate to expose the gallium indium phosphide.

According to another aspect the invention provides a photocathode structure comprising an epitaxial layer of photocathode material (which may be p-type gallium arsenide) on a major surface of a plate of single crystal gallium indium phosphide having relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material, the thickness of said layer of photocathode material being of the order of the diffusion length of electrons therein and at least part of the other major surface of the gallium indium phosphide plate being substantially free from'contact by solid material. I

" A-n embodiment of the invention will nowbedescribed, by way of example, with reference to the diagrammatic drawing accompanying the Provisional "Specification, which is an axial section of a proximity- -type image intensifier and/or converter tube (not to 'scale).*

' phosphide forexample l0 pm thick. The material of the disc 5 has approximately equal atomic percentages (ideally 49 51%)of indium and gallium so that its lattice parameter is substantially the same as that of the gallium arsenide of layer 4. The periphery of the disc 5 is supported by a circular frame 6 of gallium arsenide secured to'the envelope '1. The free surface of layer 4 activated with caesium-oxygen in known manner and. faces and is adjacent a cathodeluminescent layer '7 provided on a transparent electrically conductive layer (not shown) on the opposite end wall of the envelope 1. Supply conductors (not shown) contacting the frame 6 and the conductive layer under the layer '7 are sealed through the envelope wall 1 and serve to provide the layer 7 with a positive potential relative to the photocathode assembly 3 in operation, so that any electron image produced at the free surface of the layer 4 by an image in infra-red or visible radiation focussed thereon through the window 2 results in a corresponding luminescent image being produced by the layer 7, this being viewed through the end wall of the envelope 1.

The photocathode assembly 3 may be manufactured as follows:

A single crystal plate of gallium arsenide for example 500 um thick is polished and then has the plate 5 grown thereon by liquid epitaxy for example as described by Stringfellow in J. App. Phyqics 43 pages 3455-3460 (1972) or by vapour phase epitaxy for example by a method similar to that described by Nuese in Metallurgical Transactions 2 p. 789 et seq (March 1971). It has been found that, in the case of liquid epitaxy, provided the proportions of gallium and indium in the material used for the growth process are approximately correct (50 -50%), thermodynamic considerations favour the actual growth of gallium indium phosphide with relative proportions of gallium and indium such as to give a substantially exact lattice match with the underlying gallium arsenide (and thus also with the gallium arsenide layer 4 to be subsequently deposited on the free surface of the phosphide).

The layer 4 of p-type gallium arsenide is then provided on the free surface of the plate 5 either by vapour epitaxy as described for example by Tietjen and Amick, in J. Electrochem. Soc. 113, page 724 (1966) or by liquid epitaxy as described, for example by Panish, Sumski and l-layashi in Metall. Trans. 2, pp. 795-801 (1971). The resulting GaAs-GalnP-GaAs sandwich is then masked, for example with wax, except for the central region of the original gallium arsenide plate, and etched for example with H SO :H O :H O in the usual ratios, so that the central region of the original gallium arsenide plate is removed to leave the plate 5, layer 4 and frame 6 (the latter being formed by the remaining part of the original gallium arsenide plate).

The wax is dissolved and the layer 4 is then activated with caesium and oxygen, for example as described by Liu et al in Appl. Physics Letter 14 no. 9 pages 275 et seq (1969) and positioned adjacent the phosphor layer in the envelope 1.

If desired the gallium arsenide 4 and 6 may be replaced by gallium indium arsenide or indium arseno phosphide.

What we claim is:

l. A photocathode structure containing a photocathode material, comprising:

a. a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material, and

b. an epitaxial layer of photocathode material located on a first said major surface of said crystal, the thickness of said layer of photocathode material being of the order of the diffusion length of electrons therein and at least part of a second said major surface of the gallium indium phosphide plate being substantially free from contact by solid material.

2. An electron tube including a photocathode structure as claimed in claim 1.

3. A tube as claimed in claim 2, comprising one of an image display and converter tube.

4. A photocathode structure including gallium arsenide, comprising:

a. a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of gallium arsenide, and

b. an epitaxial layer of p-type gallium arsenide located on a first of said major surfaces, the thickness of said gallium arsenide layer being of the order of the diffusion length of electrons therein and at least part of a second of said major surfaces of said gallium indium phosphide plate being substantially free from solid material.

5. A photocathode structure as claimed in claim 4, wherein the free second major surface of the gallium arsenide layer is activated with caesium-oxygen.

6. A photocathode structure as claimed in claim 4, wherein the central portion of said second major surface of said gallium indium phosphide plate is free from solid material and a frame of gallium arsenide is present around the periphery of said second major surface.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3, 889, 143

DATED I June 10, 1975 INVENTOR(S) I JONATHAN PAUL GOWERS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE TITLE SECTION, Secticn [30] change "54387/73" Signed and Scaled this A ttes I.

RUTH C. MASON C. MARSHALL DANN Arresting Office Commissioner of Patents and Trademarks 

1. A PHOTOCATHODE STRUCTURE CONTAINING A PHOTOCATHODE MATERIAL, COMPRISING: A. A PLATE OF SINGLE CRYSTAL GALLIUM INDIUM PHOSPHIDE HAVING MAJOR SURFACES AND RELATIVE PROPORTIONS OF GALLIUM AND INDIUM SUCH THAT THE LATTICE PARAMETER THEREOF IS SUBSTANTIALLY THE SAME AS THAT OF SAID PHOTOCATHODE MATERIAL, AND B. AN EPITAXIAL LAYER OF PHOTOCATHODE MATERIAL LOCATED ON A FIRST SAID MAJOR SURFACE OF SAID CRYSTAL, THE THICKNESS OF SAID LAYER OF PHOTOCATHODE MATERIAL BEING OF THE ORDER OF THE DIFFUSION LENGTH OF ELECTRONS THEREIN AND AT LEAST PART OF A SECOND SAID MAJOR SURFACE OF THE GALLIUM INDIUM PHOSPHIDE PLATE BEING SUBSTANTIALLY FREE FROM CONTACT BY SOLID MATERIAL.
 2. An electron tube including a photocathode structure as claimed in claim
 1. 3. A tube as claimed in claim 2, comprising one of an image display and converter tube.
 4. A photocathode structure including gallium arsenide, comprising: a. a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of gallium arsenide, and b. an epitaxial layer of p-type gallium arsenide located on a first of said major surfaces, the thickness of said gallium arsenide layer being of the order of the diffusion length of electrons therein and at least part of a second of said major surfaces of said gallium indium phosphide plate being substantially free from solid material.
 5. A photocathode structure as claimed in claim 4, wherein the free second major surface of the gallium arsenide layer is activated with caesium-oxygen.
 6. A photocathode structure as claimed in claim 4, wherein the central portion of said second major surface of said gallium indium phosphide plate is free from solid material and a frame of gallium arsenide is present around the periphery of said second major surface. 